bitbake: lib/bb/utils.py: remove unnecessary preserved variables

bitbake: lib/bb/utils.py: remove unnecessary preserved variables

_ and LANG no longer need to be preserved from the external environment.
The value of _ changes between non-pseudo-wrapped and pseudo-wrapped
invocations (e.g. between "bitbake -p" and "bitbake target") and this
will currently trigger a full reparse in the absence of a whitelist
entry in BB_HASHCONFIG_WHITELIST, which is not ideal.

LANG used to be preserved in order to ensure the C locale was being
used for tools invoked by bitbake, however we now set LC_ALL in
bitbake.conf to take care of this.

Second part of the fix for [YOCTO #2600].

Acked-by: Jason Wessel <jason.wessel@windriver.com>
(Bitbake rev: 1c531dff2fb055ecab2d462027eecec3fabc2a44)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>